The Evolutionary Psychology Blog

Finger Treads

Published 10 January, 2013

I often avoid writing about stories that get picked up by the major news services because I like to find the cooler parts of the pillow, but a recent paper is about an idea that’s so fricking cool I thought I’d write about it despite the play it’s getting.

The phenomenon of interest is why fingers (and toes) get wrinkled when they have been in the water for a while. If your high school education was like mine – or if you go to sites such as wiki.answers to find such things out – then you might think that wrinkling is a side effect of the structure of the skin.

A problem with this byproduct explanation is that it predicts that wrinkling should occur even if the nerves in the fingers are severed insofar as the osmotic and structural properties of the finger don’t change if the nerves are damaged. But that’s not the case. Indeed, one suggested test for damage to the nervous system is to see if finger wrinkling in water occurs (Bull & Henry, 1977). This locates the explanation for finger wrinkling somewhere other than the skin’s structure.

The fact that this side-effect explanation runs into a problem doesn’t entail that wrinkling isn’t a side effect of something else, but in 2011, Mark Changizi and colleagues proposed that wrinkling was functional as opposed to a side effect, dubbing their idea the “rain tread” hypothesis. The general idea is illustrated with the engineering principles behind tire treads. (See Figure 2 of Changizi et al., 2011). If you watch NASCAR, then you have probably noticed that the tires they use are bald, with no treads at all. This is because a key functional property of these racing tires is that they have good traction. Bald tires maximize contact with the track, and so maximize traction. They don’t need to worry about handling under wet conditions because if the track is wet, they simply don’t race. Tires on non-racing cars reflect a tradeoff because unlike racers, normal people don’t have the luxury of taking off from work every time that it rains. The treads you see in normal everyday cars make the tires worse than they would be under dry conditions – less surface area in contact with the road – but if they were bald, driving in the rain would be a disaster. Tires with treads represent a tradeoff because they have to work in both wet and dry conditions.

Hands do, too. But, unlike tires, hands can change from bald to treaded. This is the idea behind the “rain tread” hypothesis. Changizi et al. proposed that finger wrinkling is designed to afford better grip under wet conditions. Like treads, wrinkles, they suggest, might be little drainage networks, getting water off the fingers under wet conditions to grip better. (For another recent story linking human engineering with engineering in the natural world, have a look at stories about a link between fireflies and LED lights. And if you read this blog but haven’t read Cat’s Paws and Catapults, you might consider it.)

If finger wrinkling is for grip, then it ought to be the case that wet fingers are better at gripping wet things than dry fingers are. A recent paper in Biology Letters by Kyriacos Kareklas, Daniel Nettle and Tom V. Smulders provides some preliminary data consistent with this idea. (The Guardian has a little video on the paper.)

Kyriacos et al. had twenty subjects perform a task in which the subjects had to move objects, mostly marbles, from one container to another in as little time as possible. Half the time they had to move the objects with normal fingers; the other half their fingers were wrinkled from being held under water for half an hour. (That is, the fingers, not the whole subject.) In one condition, the objects were dry; in the other condition, the objects were taken out of a container of water.

It took subjects a couple of minutes, on average, to do the task. For the dry case, there was no difference between moving the objects where they were wet and when they were dry. That is, there was no advantage or disadvantage to having wrinkled fingers for dry objects. Moving wet objects with wrinkled fingers gave a small but significant advantage of twelve per cent.

The lack of a difference in the dry conditions raises the question, why aren’t fingers always wrinkled? As the authors point out, grasping isn’t the only consideration. Perhaps there are disadvantages to having wrinkled fingers for purposes other than grasping, such as a loss of sensitivity.

Now, I should note that this work was only done with young adults in a Western setting, so we should be very cautious about generalizing. Perhaps in the highlands of New Guinea, finger treads reduce the ability to manipulate submerged objects, or perhaps the phenomenon doesn’t occur at all.

In any case, these data are consistent with the rain tread hypothesis, though of course more work needs to be done. Still, the rain tread hypothesis seems to require that there be some advantage of this sort, and these initial data are encouraging.